Williams driver Felipe Massa lamented a 'difficult day' as he struggled for grip in his Williams FW37 at the Brazilian Grand Prix.
Home racer Massa suffered several moments during Friday's practice sessions at Interlagos and was left down in 10th place at the end of running.
"It was a very difficult day," he said. "The car was unstable and I struggled to get the grip I wanted at the rear.
"I don't think I had one clean lap without losing the rear at some point.
"The surface offers low grip but very high tyre degradation, so the car was sliding a lot.
"We have a lot of work to do to tonight to make the car a bit easier to drive and more competitive for qualifying tomorrow."
A more upbeat Valtteri Bottas was sixth fastest, despite a spin at the downhill Pinheirinho corner.
"We got a lot of information about the tyres today, which was the main priority," he said.
"We got the soft and medium compounds to work well and the long runs felt consistent, so I'm really hopeful that we can be strong this weekend.
"It was more slippery than last year, but it is the same for everyone and I was quite comfortable."